Add version string to lune-utils crate

Cargo.lock

@@ -1644,6 +1644,7 @@ name = "lune-utils"
 version = "0.8.3"
 dependencies = [
  "mlua",
+ "once_cell",
 ]
 
 [[package]]

crates/lune-utils/Cargo.toml

@@ -12,3 +12,5 @@ workspace = true
 
 [dependencies]
 mlua = { version = "0.9.7", features = ["async"] }
+
+once_cell = "1.17"

crates/lune-utils/src/lib.rs

@@ -1,5 +1,7 @@
 #![allow(clippy::cargo_common_metadata)]
 
 mod table_builder;
+mod version_string;
 
 pub use self::table_builder::TableBuilder;
+pub use self::version_string::get_version_string;

crates/lune-utils/src/version_string.rs

@@ -0,0 +1,66 @@
+use std::sync::Arc;
+
+use mlua::prelude::*;
+use once_cell::sync::Lazy;
+
+static VERSION_STRING: Lazy<Arc<String>> = Lazy::new(create_version_string);
+
+/**
+    Returns the current Lune version string, in the format `Lune x.y.z+luau`.
+
+    This version string is strongly guaranteed to follow the above
+    format and may safely be used for parsing & version comparisons.
+*/
+#[must_use]
+pub fn get_version_string() -> Arc<String> {
+    Arc::clone(&VERSION_STRING)
+}
+
+fn create_version_string() -> Arc<String> {
+    // Extract the current Luau version from a fresh Lua state / VM that can't be accessed externally.
+    let luau_version_full = {
+        let temp_lua = Lua::new();
+
+        let luau_version_full = temp_lua
+            .globals()
+            .get::<_, LuaString>("_VERSION")
+            .expect("Missing _VERSION global");
+
+        luau_version_full
+            .to_str()
+            .context("Invalid utf8 found in _VERSION global")
+            .expect("Expected _VERSION global to be a string")
+            .to_string()
+    };
+
+    // Luau version is expected to be in the format "Luau 0.x" and sometimes "Luau 0.x.y"
+    assert!(
+        luau_version_full.starts_with("Luau 0."),
+        "_VERSION global is formatted incorrectly\
+        \nFound string '{luau_version_full}'"
+    );
+    let luau_version_noprefix = luau_version_full.strip_prefix("Luau 0.").unwrap().trim();
+
+    // We make some guarantees about the format of the _VERSION global,
+    // so make sure that the luau version also follows those rules.
+    if luau_version_noprefix.is_empty() {
+        panic!(
+            "_VERSION global is missing version number\
+            \nFound string '{luau_version_full}'"
+        )
+    } else if !luau_version_noprefix.chars().all(is_valid_version_char) {
+        panic!(
+            "_VERSION global contains invalid characters\
+            \nFound string '{luau_version_full}'"
+        )
+    }
+
+    Arc::new(format!(
+        "Lune {}+{luau_version_noprefix}",
+        env!("CARGO_PKG_VERSION")
+    ))
+}
+
+fn is_valid_version_char(c: char) -> bool {
+    matches!(c, '0'..='9' | '.')
+}